Around 150,000 people have been evacuated

The official Xinhua news agency said seven tanks were leaking fumes at the plant in the city of Chongqing.

Residents within a kilometre were moved after the leak began on Thursday. Three tanks have since exploded, prompting more evacuations, the agency reported.

In December, Chongqing was rocked by a gas field blast that killed 234 people.

Workplace safety standards are low in China, leading to thousands of deaths and many more injuries each year.

Inhalation of 2.5 milligrams of chlorine gas is believed to be enough to cause death, according to the AFP news agency.

'Antiquated equipment'

The gas began to leak from the Tianyuan Chemical Industry Plant in the densely populated Jiangbei district of Chongqing on Thursday evening, residents and Xinhua said.

"Around 0200 [on Friday], the guards woke up all the residents in this housing compound and told them to leave," an employee told AFP.

"They were taken to cafeterias of government offices and sports facilities and stadiums."

Reports said efforts were being made to neutralise the leaks by discharging the chlorine tanks into water.

But by Friday evening, three of the tanks had reportedly exploded, which forced more people to be evacuated.

"Since the explosions of chlorine gas, so far 150,000 people have been evacuated," Xinhua was quoted as saying by AFP.

In Jiangbei district, 80,000 people were relocated, while another 68,000 people in surrounding areas were also evacuated, Xinhua said.

Over 100 soldiers have been sent to the scene to help to relocate residents.

"Initial evaluation shows the explosions were caused by antiquated equipment and improper operation," the director of the rescue team said.